It is well known that the refractive properties of the cornea can be altered by the selective removal of corneal tissue. For example, a myopic condition of the eye can be corrected by selectively removing corneal tissue from the central portion of the cornea. Similarly, a hyperopic condition can be corrected by selectively removing corneal tissue within a peripheral ring surrounding the central portion of the cornea.
With this in mind, a general knowledge of the anatomy of the cornea is helpful to appreciate the problems that must be confronted during refractive corrections of the cornea. In detail, the cornea comprises various layers of tissue which are structurally distinct. In order, going, in a posterior direction from outside the eye toward the inside of the eye, the various layers in a cornea are: an epithelial layer, Bowman's membrane, the stroma, Descemet's membrane, and an endothelial layer. Of these various structures, the stroma is the most extensive and is generally around four hundred microns thick. Additionally, the healing response of the stromal tissue is generally quicker than the other corneal layers. For these reasons, stromal tissue is generally selected for removal in refractive correction procedures.
In greater detail, the stroma of the eye is comprised of around two hundred identifiable and distinguishable layers of lamellae. Each of these layers of lamellae in the stroma is generally dome-shaped, like the cornea itself, and they each extend across a circular area having a diameter of approximately nine millimeters. Unlike the layer that a particular lamella is in, each lamella extends through a shorter distance of only about one tenth of a millimeter (0.1 mm) to one and one half millimeters (1.5 mm). Thus, each layer includes several lamellae. Importantly, each lamella includes many fibrils which, within the lamella, are substantially parallel to each other. The fibrils in one lamella, however, are not generally parallel to the fibrils in other lamellae. This is so between lamellae in the same layer, as well as between lamellae in different layers. Finally, it is to be noted that, in a direction perpendicular to the layer, each individual lamella is only about two microns thick.
One technique for altering the refractive properties of the cornea involves the use of a pulsed laser beam to photoablate stromal tissue. In this technique, a pulsed laser is focused beneath the anterior surface of the cornea to photoablate tissue within the stroma. Heretofore, it has been suggested that the optimal photoablation of tissue with minimal side effects can be obtained using a laser beam having a pulse duration of 100 femtosecond (fs) focused to an ablation spot size of approximately 10 μm with a pulse energy approximately equal to the ablation energy threshold. However, with these parameters, a typical refractive procedure (e.g. a procedure involving the ablation of an area having an approximate diameter of 6.5 mm) would require an undesirably long scan time. Specifically, a single pass of the laser beam over an area this size may require approximately 400,000 pulses, and further, the corrective procedure may require several passes. Thus, for a typical laser beam having a pulse repetition frequency of approximately 10 KHz, each pass would take almost 40 seconds.
It is to be appreciated that procedures requiring a lengthy scan time (e.g. 40 seconds or more) can pose a number of serious problems. One such problem involves the movement of the eye during a scan. To overcome eye movement, eye restraint is often used. Unfortunately, restraining the eye is only somewhat effective and long periods of eye restraint can cause serious discomfort for the patient. In addition to eye movement, patient blinking is another factor that must be considered during a corneal laser procedure. Each time a patient blinks, a new tear film is deposited on the anterior surface of the cornea. Each tear film affects the optical path of the laser beam in a slightly different manner, affecting the precision of the operation. Thus, it is preferable to perform an entire laser scan with a single tear film, if possible. Typically, 10 seconds is about the maximum time that a patient can restrain from blinking, thus it is preferable to complete an entire laser scan in less than about 10 seconds.
In addition to requiring an unacceptably long laser scan, operating at or near the ablation energy threshold has other drawbacks. Specifically, operating at or near the ablation energy threshold is non-optimal because statistical fluctuations of the tissue ablation process are more pronounced (compared with ablation at energies significantly above threshold) leading to ablation non-uniformities. These ablation non-uniformities, in turn, can create undesirable refractive inhomogeneities.
In all surgical procedures, damage to non-target tissue is to be avoided. During photoablation of target tissue, nearby (non-target) tissue is heated. Although some heating of non-target tissue can be accommodated without damage, excessive heat must be avoided. In greater detail, for stromal tissue, a temperature rise of about 3° C. can be tolerated without long-term cell damage. In contrast, temperature increases of between about 8° C. and 23° C. can result in tissue shrinkage, cell denaturation, loss of cell function and coagulation.
During photoablation, a series of gas bubbles are formed as the laser beam is scanned through the stroma. If the ablation sites are created too closely together and the bubbles are large, the bubbles may overlap. Typically, it is this overlap that is responsible for most of the heat damage to non-target tissue. Generally, ablation using a relatively large pulse energy results in a relatively large bubble, and conversely, ablation using a relatively small pulse energy results in a relatively small bubble. With this in mind, one way to prevent bubble overlap and its associated heat damage is to use relatively low pulse energies to create relatively small bubbles. However, as indicated above, relatively low pulse energies can lead to ablation nonuniformities and unacceptably long procedure times.
In light of the above, it is an object of the present invention to provide methods and apparatuses suitable for photoablating a relatively large amount of targeted stromal tissue in a relatively short scan time without heating non-target tissue to harmful temperatures. It is yet another object of the present invention to provide methods and apparatuses for photoablating stromal tissue at pulse energies sufficient to prevent non-uniform ablation and with little or no adverse side effects. Another object of the present invention is to provide stable and efficient methods and apparatuses for photoablating stromal tissue. Yet another object of the present invention is to provide methods and apparatuses for changing the refractive properties of a cornea which are easy to use, relatively simple to implement, and comparatively cost effective.